I believe the FF's take elements from every religion. Much like how every religion takes elements from each other. It's only natural that a fantasy story that often has it's own mention of religions would be influenced by those of the creators worlds. I don't agree with the analogies you've applied to them, such as your IX theory and much of the other ones you detailed, but I do admit that the worlds religions are reflected in the Final Fantasy series. I don't think it all stems from one though, Christianity, as you are implying.